<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

<Style TargetType="{x:Type Button}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type Button}">
                <Grid>
                    <Ellipse Fill="{TemplateBinding Background}"
                             Stroke="{TemplateBinding BorderBrush}"/>
                        <ContentPresenter HorizontalAlignment="Center"
                                          VerticalAlignment="Center"/>
                </Grid>            
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

<DataTemplate DataType="x:String">
  <Border BorderThickness="5" BorderBrush="Red">
  <TextBlock Text="{Binding}" />
  </Border>
</DataTemplate>

</ResourceDictionary>